The Crooked Ring (The Saint TV Series)

Plot
Steve Nelson's an up-and-coming boxer, but his girlfriend approaches Simon because she's afraid of his next opponent; a fighter known as the Masked Angel, who killed a man in a fight. Although there was no evidence of foul play, Templar's suspicious of the Angel's manager, the more-so after a man who had offered to spy on him is found dead and Templar's shot at.

Revealing mistakes

 * The farthest two rows of spectators in the wide shot of the boxing ring were all obviously cardboard cutouts.

Origin
This episode was adapted from the novella The Masked Angel from the 1948 book Call for the Saint.